This is a division of copending U.S.A. Patent Application Ser. No. 337,566 filed Mar. 2, 1973 now U.S. Pat. No. 3,854,818.
This invention relates to electrical signal amplitude peak detecting arrangements. The invention relates more particularly to improvements in peak detecting means employed with analytical instruments.
This invention is described herein with reference to its application to an atomic absorption spectrometer for purposes of illustration; the circuit arrangement of this invention is adapted and intended for use with any type of instrument in which means for detecting electrical signal peak amplitudes, and for compensating error signal components, could beneficially be utilized.
The atomic absorption spectrometer is a known form of quantitative analytical instrument in which a sample is atomized in the presence of light energy of a limited predetermined spectrum. This spectrum corresponds to the absorption spectrum characteristics of a constituent material in a sample composition under analysis.
Absorption of light by the atomized constituent material is sensed by photodetection means and a corresponding electrical indication is thereby generated. The instrument includes means for providing an analog output signal having a peak amplitude which is representative of the absorbance and thus of the concentration of the material. This output signal is then applied to a display or a recording means such as a chart recorder which provides an indication from which the instrument operator can determine the concentration of the material in the sample composition.
It is at times desirable to provide an instrument output signal for use with digital equipment. This is desirable, for example, when the instrument output is to be analyzed by data handling equipment. Similarly, a digital signal is desirable in order to facilitate analysis by an instrument operator through the use of digital output displays and printers. One technique for the conversion of an analog signal of this type to digital form comprises sensing the peak amplitude of the analog instrument output signal and converting this amplitude into an equivalent digital representation. The accuracy of the digitized signal is, of course, largely dependent upon the accuracy with which the peak detection is accomplished.
Various factors such as changes in characteristics of components within the instrument and electrical noise contribute to inaccuracies in the peak amplitude of the output signal. These factors present themselves cumulatively as a deviation in the generally steady or relatively slowly varying baseline of the output signal from a desired reference level. Because these error deviations are of relatively long-term duration, the undesired baseline deviation can be readily corrected. The instrument operator accomplishes the correction by the adjustment of output signal zeroing or compensating means. The resulting baseline correction thus provides a substantially error free signal for conversion into digital form.
Improvements in the atomic absorption instrument, however, have resulted in the generation of a relatively short-term output signal which at times is preceded and followed by relatively short-term interference components. For example, although it is desirable to utilize a heated graphite atomizer form of furnace, it is known that this type of atomizer generates a short sample absorption output signal peak and a relatively short-term interference component which immediately precedes and follows the output signal peak. These interference components alter the peak amplitude of the signal with respect to the reference level and thereby introduce an error into the signal. In view of the relatively short-term nature of these error-introducing components, it is not feasible and generally not possible for the instrument operator to readjust the zeroing means in order to compensate for these short-term deviations from the reference level.
Accordingly, it is an object of this invention to provide an improved method and means for detecting the peak amplitude of an electrical signal.
Another object of this invention is to provide a peak detecting method and means adapted for compensating for relatively short-term interference or error signal components which accompany the occurrence of an information signal.
Another object of the invention is to provide a peak-detecting means adapted for compensating for relatively short-term positive or negative interference or error signal components.
Another object of the invention is to provide an improved peak detecting method and means adapted for compensating for repeatable, relatively short-term interference or error signal components of substantially constant amplitude.
In accordance with the peak detection method of this invention, a repeatable error signal of relatively short-term duration is derived from an information signal source and is applied to an input terminal of a peak detection arrangement. The peak amplitude of this error signal is detected and a representation thereof is stored. An information signal and the repeatable error signal are subsequently coincidentally derived from the information signal source and are applied to the input terminal of the peak detection arrangement. The peak sum of these signals is detected and is altered in accordance with the amplitude of the stored representation in order to provide a peak detected information output signal from which the amplitude of the short-term error signal is substantially removed.
In accordance with features of the peak detection arrangement of this invention, a circuit means is provided for detecting and providing a signal V.sub.d representative of the peak amplitude of an input signal V.sub.in which is applied thereto. Circuit means are provided for selectively storing a representation of the amplitude of the signal V.sub.d and for altering the detected peak amplitude of a subsequently applied input signal in accordance with the amplitude of the stored representation to provide an output information signal V.sub.O which is corrected for error or interference components. In accordance with more particular features of the peak detection arrangement of this invention, the storage and amplitude altering circuit means is adapted for automatically combining the peak amplitude of a stored error signal V.sub.cst in phase opposition with the peak detected signal V.sub.d corresponding to a subsequently applied input signal to thereby provide an output information signal V.sub.O from which repeatable, short-term error signal components are substantially removed.
In accordance with further features of the invention, a peak detection circuit arrangement comprises circuit means for detecting error signal components of relatively positive and negative polarity of the error signal component of larger absolute amplitude, and for effecting combination of the stored representation with a subsequently detected composite signal having information and error signal components in a manner for substantially removing the error signal component of the composite detected signal.